1. Technical Field
This invention relates generally to precoat filters and to precoat septa, and more particularly to a new and improved precoat septum utilizing an improved winding formed of yarns produced from continuous filaments. Such filters and septa are useful in industrial filtration.
2. Prior Art
A precoat filter comprises a particulate filter medium, such as an ion exchange resin or a diatomaceous earth or comparable material, deposited or "precoated" onto a supporting septum. In operation, a fluid is passed through the particulate precoat where undesirable materials are taken up by the filter medium. The filtrate passes through the septum and out of the filter.
In some cases, the septum is provided in the form of a spiral winding of yarn applied over a porous tubular core. The septum of the present invention is of this type, and the following discussion and disclosure is addressed to that context.
In such precoat septa, the yarn winding does not serve as a filter medium, but rather as a permeable surface upon which a layer of materials such as powdered ion exchange resin or diatomaceous earth are deposited. Such materials layered onto the outer surface of the yarn winding ionically attract and/or retain the particulate matter as the liquid influent flows into and through the yarn medium. When the outer layer of precoat filter medium become sufficiently loaded with solids that flow therethrough becomes restricted, the septum is backwashed to dislodge the particulate filter medium, and its load of debris, from the septum surface and wash it away. Thereafter, a new powdered ion exchange resin coating is applied to the winding surface, and the septum is reused until it again becomes loaded. It is intended therefore, that such septa be backwashed and reused many times.
The yarns utilized in precoat septa and the technique for winding the yarns onto the support core to produce an effective septum must be carefully selected and matched in order to achieve the degree of permeability, precoat retention, backwashing effectiveness, and service life desired. The yarns most often utilized have been spun yarns made of staple fibers; i.e., typically natural fibers such as cotton or chopped man-made fibers, either of which are composed of short fibers, typically less than three inches in length, twisted together to form a continuous length of yarn, as is essential to effect a useful degree of cohesiveness to the fibers and a degree of tensile properties in the resulting yarn. Continuous filament yarns have not been used in precoat septa, generally because they lack the numerous fiber ends which extend from surface of spun staple yarns.
Precoat septa are similar in terms of design and construction to spiral wound tubular filter cartridges but are also different in some important respects.
Spiral wound tubular filter cartridges are well known in the art, and typically comprise a perforated tubular support core having a winding of yarn wrapped thereon which serves as the filter medium.
Unlike filter cartridges, where particle entrapment must be effected throughout the depth of the yarn winding filter medium, it is essential in precoat septum applications that particle entrapment be minimal within the body of the yarn winding. That is to say, in a cartridge filter, the cartridge is normally utilized until it becomes plugged or nearly plugged, and then it is discarded. Therefore, particle entrapment throughout the entire depth of the filter medium is essential to optimize the filter's usefulness. In the case of the precoat septa, however, backwashing is an essential requirement so that the device can be cleaned and reused over and over again. Ideally, therefore, particle entrapment within the depths of the winding is not desired, since particles entrapped within the depth do not wash out effectively. Accordingly, the yarn winding on a precoat septum is preferably wound to assure a tighter pore size, relative to the dimensions of the particulate filter medium, to prevent particulate matter from entering the depth of the yarn winding.
In practically all wound cartridge applications, the utilization of staple fiber yarns has generally been deemed essential to effect the filtration desired. This is because the yarn is characterized by a high degree of bulk or surface "hairiness" caused by the protrusion of the ends of short fibers from the main body of yarn. Accordingly, when the yarn strands are wound onto the filter support core to provide a medium consisting of a plurality of adjacent and overlaying yarn strands, the short lengths of fiber extend into and across the open spaces between the adjacent strands of yarn.
In filter cartridges where the yarn winding functions as the filter medium, the surface hairiness serves the additional function of entrapping the particulate matter so that the projecting fiber ends are themselves a primary filtering element. As a result, the winding ratio can be adjusted to provide a more open yarn pattern in the winding which, in turn, provides higher flow rates of filtrate through the winding. These same concepts and materials have also employed in the design and manufacture of conventional precoat septa despite the fact that these devices are intended to serve functions distinct and separate from those of a filter cartridge as previously described.
The hairy characteristic of the yarn, while still deemed essential to prevent overpacking (i.e., a tight winding pattern) even in precoat septa, does create disadvantages in that the hairiness tends to prevent an even coating of the powdered ion exchange resin or other precoat material, and further tends to interfere with the subsequent complete removal of the spent precoat from the winding surface during the backwashing operation. In addition, experience has shown that such yarns do not have sufficient tenacity and mechanical toughness to withstand the rigors of repeated backwashing to the extent desired, so that the yarns are easily frayed and surface hariness is increased-creating an undesirable, non-uniform surface which prevents the formation of a uniform precoat and may permit shedding of staple fibers from the septum winding into the filtrate and eventual bleedthrough of the precoat material. Such shedding of septum yarn fibers contaminates the filtrate and is unacceptable in most filtration operations. Bleedthrough of the precoat, of course, is a serious failure. It reduces the filtering and treatment effect and cannot be tolerated to any measurable degree. Similarly, a non-uniform precoat thickness permits the passage of fluid which has not been completely filtered nor treated, thereby defeating the function of the septum. Under any of these conditions, filtration must be stopped, the filter vessel opened, and the septum replaced. Since depth filters are not ordinarily backwashed, these considerations are irrelevant to their context of operations and their structure.
Additionally, yarns spun of staple fibers have an undesirable degree of non-uniformity with respect to strength, weight and surface characteristics, making it particularly difficult to manufacture a precoat septum with consistent performance characteristics and quality.
Yarns produced from continuous fibers or filaments also do not normally provide the balance of permeability, uniform precoat formation, precoat retention, and backwashability desired in precoat septa. This is because the continuous filaments from which the yarns are produced are not short strands, but rather are continuous strands, being as long or longer than the yarn itself, are longitudinally oriented, tending to be reasonably straight and highly parallel, and do not have the protruding fibers, i.e., surface "hairiness", that is essential to achieve particle retention and adequate permeability. Due to the lack of protruding fibers and bulk, such yarns are not particularly effective in retaining the precoat material on the surface of the septum. If the winding pattern is adjusted to reduce the size of the apertures between adjacent yarn strands to compensate for the lack sufficient hairiness, and to thereby provide for adequate retention, the permeability of the wound structure becomes excessively restrictive to fluid flow. The utilization of continuous filament yarns, however, is desirable to some extent since durability and effective backwashing performance is obtained.
As a solution to the above problems, the utilization of a "textured" continuous filament yarn has been employed in spiral wound depth filter cartridges. Such yarns are characterized by fibers which have been given a "kink", "coil", "figure", "crimp", or some such geometric form other than straight by any one of a number of known processes, such as "false twist" "knit-deknit" "stuffer box" "air jet texturing" and other such processes to provide a "textured" or "bulked" characteristic to the resulting yarn. Such bulked or textured yarns, however, are not suitable for precoat septa. In the manufacture of precoat septa, high winding tension is essential to produce winding with sufficient surface hardness and structural integrity to withstand the stress of repeated backwashing. The tensions utilized tend to pull-out and straighten the textured fibers, so that the yarn, when wound on the support core resembles those of straight, untextured fibers, exhibiting the overpacking tendency and permeability limitation described above. The filaments of the yarn will tend to spread to form a homogeneous filamentary layer; see Tomita, et al., U.S. Pat. No. 4,761,231. Such a structure has several attractions for depth filters, but are not desirable or acceptable for precoat septa. First, and most importantly, the hydraulic resistance of the winding is materially increased by the close packing and the spreading of the filaments of the yarn, reducing filtrate flow or requiring higher pressure differentials across the winding. Next, the strength of the winding, particularly under backwash conditions may be compromised. Spiral wound depth filters are generally not backwashable, and are normally discarded when fouled, and have no need to withstand the rigors of backwash procedures.